1. Field of the Invention
The present invention relates to the valves associated with a commercial scale sulfur recovery process converting H.sub.2 S and SO.sub.2 to elemental sulfur. More specifically, it involves an improved method of preventing sulfur losses caused by leakage while protecting the valves from the corrosive environment.
2. Description of the Prior Art
In a contemporary commercial scale sulfur recovery plant, such as a Claus plant or Claus plant followed by a Cold Bed Adsorption reactor or the like, it is not uncommon to have as many as a dozen or more valves for switching the flow. These valves frequently are several feet in diameter and designed to withstand high concentrations of corrosive vapors such as H.sub.2 S and SO.sub.2 at temperatures which range from about 250.degree. F to 650.degree. F or even higher. With the advent of strict environmental emission laws, the degree of sulfur recovery has increased from the historic 80-90% recovery to 98-99% or better with commercial performance and performance guarantees being determined and specified to tenths of a percent. To achieve these conversion levels, the final reactor or reactors of a Claus plant are now being frequently operated at temperatures below the dew point of the sulfur vapors which ultimately requires high temperature regeneration of the reactor as exemplified in U.S. Pat. No. 4,035,474. Consequently, some of these very large valves have to give tight shutoff at the lower temperature as well as at the higher temperature. To further emphasize the problem quantitatively, a one-half percent leakage across certain valves in the Cold Bed Adsorption process can reduce recovery efficiency sufficiently to lower overall sulfur yield as much as 0.2%. Thus, the use of many conventionally constructed valves at critical points in the flow scheme is virtually prohibited.